In an autumn frame of mind

September 23, 2010

Well, how do you recognize autumn? You notice your alarm going off in the darkness of the morning hours, unlike in July, when it was light enough to eat breakfast outside. You can't grill outside at 9 p.m. anymore, and daylight savings is only partially to blame. The earth is getting closer to the sun, but the sun is moving further and further south in the sky and becoming increasingly rare. The Autumnal equinox is Sept. 23, when the sun was directly over the equator.

The seasonal changing of day length is a signal for us to carve pumpkins, and a signal for plants to prepare for winter. Shortening day length (perceived by plants as lengthening night length) is a reliable predictor of lower temperatures, so shorter days serve as a good signal to begin preparing for winter.

While the change of color and subsequent leaf loss seems to coincide with the cool, crisp days and nights of fall, it is actually induced with the shortening length of the day for many species. This is a photoperiodic response, or a response to the length of the day. Photoperiod also plays a role in breaking dormancy and flowering for many species.

Temperature also plays an important role in promoting dormancy in plants. As the temperature decreases, exposure to the gradually decreasing temperatures enables perennials and biennials to acclimate to the temperatures, which increase their cold hardiness.

One way cold hardiness is increased in plants such as trees is with the presence of sap. Sap is water with dissolved sugars that flows through the phloem tissue of the plant. Dissolved sugars in the water decreases the freezing point of the solution below that of plain water. The xylem, water-conducting tissue, has water withdrawn because it would be vulnerable to damage from freezing water as it expands.Individual living cells are protected from freezing as well. So-called "antifreeze proteins" may protect the cells from ice crystal damage. Also, when ice is formed externally to plant cells, it draws water from within the cell.

This effectively concentrates the solution within the cell, which again, decreases the freezing point and protects the contents of the cells.

An important part of winter dormancy is to protect next year's leaves and growing tips, or meristems. With the shortening days of the late summer and early fall, minute leaves on very compressed stems are formed near the axils of leaves and at the tip of stems or underground storage structures in herbaceous perennials. These are the buds that will break next spring and provide next year's growth. Bud scales will primarily function in three ways: one, to keep the tissues warm; two, to prevent oxygen from reaching the buds; and three, to keep the low humidity and winds from desiccating the tissue.

Once next year's buds are ready, both deciduous woody and herbaceous plants must loose their leaves. This is a two-stepped process starting with senescence and followed by abscission. Senescence and abscission are controlled by plant hormones, or phytohormones.Next to the separation layer is a group of cells called the protective layer. This layer provides protection to the area of the stem where the leaf was attached.

So wake up late this weekend and see the sunrise, get out your jacket and enjoy the fall colors, and enjoy a 7:30 p.m. dinner by candlelight.Plants will be working hard to ensure they'll be around after winter to do it all over again.